The present application relates to a bone anchoring device.
Various designs of polyaxial bone screws are known wherein each design has particular characteristics and wherein different locking principles are used.
DE 43 05 576 C1 describes a polyaxial bone screw having a screw element and a receiving part and a pressure element acting onto the head of the screw element to press it down against the seat in the receiving part in order to lock the rotational position of the head. The bone screw usually is preassembled so that the screw element is screwed into the bone with the receiving part mounted on the screw element.
Other bone screws are known wherein the head is clamped from the side to lock the rotational position. EP 0 242 708 A2 describes a bone screw with a receiving part consisting of two spherically-shaped halves pressing from two lateral sides onto the spherical head. The halves are held together in their lower portion by a ring.
U.S. Pat. No. 5,672,176 discloses a bone screw with a receiving part with a conically shaped seat and a conically-shaped pressure element which exerts pressure onto the head from above and from the side. If the cone angle has a value lying within a specific range self-locking of the pressure element within the receiving part takes place which allows to preliminary lock the head within the receiving part while the rod is still moveable in order to allow the adjustment of its position.
EP 0 951 245 B1 describes a bone screw with a receiving part having a conically-shaped cavity accommodating the head wherein a spring chuck is provided in the cavity which is downwardly and radially compressible to clamp the head. With this bone screw it is possible to click the receiving part onto the head of the screw element which allows to screw the screw element into the bone and to connect it to the receiving part thereafter.
U.S. Pat. No. 5,728,098 discloses a bone screw for connection to a spinal rod comprising a screw element and a receiver member which has slits provided at the bottom of the rod receiving channel and wherein two locking rings made of a shape memory alloy are provided one at the lower side of the receiver member and one of the upper side. The locking rings contract about the portions of the receiver member when the temperature is elevated so that the rod is clamped in the channel.
U.S. Pat. No. 5,549,608 discloses a polyaxial bone screw with a screw element with a spherical head and a coupling element to couple the screw element to a spinal rod. The coupling element has a tapered lower portion including a slotted interior chamber in which the spherical head is initially polyaxially disposed. The coupling element further has a recess for receiving the head. In addition, a locking ring surrounding the lower portion of the coupling element and a cylindrical rod securing sleeve which fits over the coupling element are provided. A locking nut is used to exert pressure onto the rod securing sleeve. The head is locked in the interior chamber by means of the locking ring which is pressed down by the rod securing sleeve. U.S. Pat. No. 5,575,792 discloses a similar device using a hook instead of a bone screw. In contrast to the afore-mentioned bone screw the rod securing sleeve is omitted and the rod directly presses onto the locking ring. The locking ring has to be placed onto the coupling element from above. It is not secured against escaping towards the upper end and against rotation when the rod is not inserted. Furthermore, the size of this known bone anchoring device is quite large as the locking ring and the locking nut extend substantially outward from the diameter of the coupling element.
Based on the foregoing, there is a need to provide an improved bone anchoring device which comprises a minimum number of parts and which has a small size while simultaneously providing a safe locking and can be used as a modular system.